1. Field of the Invention
The present invention relates to electrical and electronic circuits and systems. More specifically, the present invention relates to microwave Class E power amplifiers.
2. Description of the Related Art
Highly efficient microwave and radio frequency (RF) high power amplifiers are required for many commercial and military applications. Typical applications include wireless local area networks, cellular phones and telecommunication systems, as well as advanced airborne active phased array radar systems. Class-E power amplifiers are high-efficiency amplifiers that would be useful for these and other applications.
Class-E amplifiers are part of the “switching mode amplifiers” such as class D, E, F, etc. These types of amplifiers include a transistor that operates as a perfect switch with no overlapping voltage and current waveforms at its output terminal, thereby ideally dissipating zero DC power. Other classes of amplifiers such as class A, A/B and C, operate as current-sources with overlapping voltage and current waveforms and hence dissipate DC power, leading to a lower efficiency.
A Class-E amplifier typically consists of a single transistor and a resonant load network. The function of the load network is to shape the voltage and current waveforms at the active device output terminal to prevent simultaneous high voltage and high current in the transistor, thereby minimizing DC power dissipation. The active device acts as a switch, driven by an RF input signal to “ON” and “OFF” conditions. The operating point of the device is such that the device is either OFF (in the pinched-off region) or ON (in the linear region). Under an ideal switching operation condition, output voltage and current waveforms at the device output terminal do not exist simultaneously and, therefore, the energy dissipated within the device is zero, yielding a 100 percent theoretical power conversion efficiency.
Present class-E high power amplifiers (HPAs) have limited useful bandwidth due to their highly tuned load circuits. Class-E HPAs have generally been used at audio, HF and UHF frequencies for applications such as audio HI-FL systems, Ham radios and high power plasma generation where narrow frequency bandwidth are required. In recent years, the published work on monolithic class-E power amplifiers has been limited to narrow band (less than 500 MHz) RF frequencies covering the hand set cell phone market.
In addition, conventional class-E amplifiers have limited power outputs. Class-E amplifiers are typically implemented using single stage designs. In order to increase gain, the size of the amplifier's active device is increased. Increasing the active device, however, reduces the frequency range of the amplifier due to the increased capacitance at the input of the device.
Hence, a need exists in the art for an improved Class-E amplifier offering simultaneous high power and high power added efficiency (PAE) over a broader frequency range.